This dissertation contributes to practice-ethnographic educational research and focuses on the phenomenon of subjectivation through performance assessment. It examines alternative assessment practices in secondary school classrooms that aim to evaluate performance without grades in order to promote individual learning development. Using the practice ethnographic approach, the unintended power effects of these assessment practices are analysed. Using four different assessment practices, power dynamics and self-relations of students and teachers are examined to understand how these practices influence the formation of self-relations and how an assessment regime is unfolded in the classroom by processing an increasing responsibiliation of students for performance. The study emphasises the importance of a differentiated investigation of assessment practices and concludes with an outlook on possible future research approaches.